34 may/june 2011 www.fluidpowerjournal.com | www.ifps.org

Vacuum Measurement

Dan Pascoe

Often missing from a vacuum system is something to measure the vacuum level such as a vacuum switch or a vacuum gauge such as shown in Fig. 1.

A vacuum gauge is a low-cost option for trouble-shooting and production efficiency. Vacuum pumps are selected based on two fundamental criteria: flow rate and vacuum level. The flow rate is determined by understanding the vacuum level required. Any vacu-um pump or generator (air-powered venturi) will give you the performance shown in the manufacturer’s catalogue. However, the flow rate of a pump or gen-erator will determine if the system can maintain the desired vacuum level in the actual application.

For example, if there is a leak in the vacuum cir-cuit, the vacuum level might fall because the pump or generator cannot “keep up” with the loss. Imag-ine sucking through a straw with your hand on the other end. You can create a vacuum easily, crushing the plastic straw with ease. If the straw splits, main-taining a vacuum is a lot harder. That’s why a larger vacuum flow will be required to offer extra capacity to compensate for this leak.

If a process requires a certain vacuum level to fulfill is production design needs, a vacuum gauge should be installed to offer the user a visual indication of sys-tem performance.

Obvious enough. However, in vacuum lifting with vacuum cups, more often than not a vacuum gauge is not installed. There is rarely a good reason for this. The standard vacuum gauge as shown in Fig. 1 is a very low-cost solution to vacuum measurement. A simple bourdon tube design, normally manufactured in brass for all wetted parts (wetted parts refer to the parts of the gauge in contact with the media—oil, air, etc) with center back or stem-mounted threads

that connect directly or indirectly to the vacuum line. These gauges offer an analogue reading of the vacuum level at that point in the system.

Placement of these gauge(s) in a vacuum sys-tem is important. Many vacuum generators offer a vacuum gauge integral to or connected to the main vacuum port. In centralized vacuum systems where one venturi is being used for several vacuum cups, the gauge reading at the venturi could be significantly different from the reading taken at the vacuum cups due to length of tubing or poor choice of fittings and other pipe connecting components. Quite often in a centralized system, the venturi is far from the work-ing end of a vacuum-handling system, the vacuum cups. Having a gauge or indeed multiple gauges in the vacuum circuit offers considerable insight into what’s happening in the system and also significant problem-finding advantages.

One of the questions often asked is, “Why won’t my cups pick up the product?” I always ask what vac-uum level they are achieving and 90% of the time the answer is, “I don’t know.” Without that information, a prognosis is certainly a more complicated exercise, particularly if this is a “phone-in” question. Vacuum level can quickly determine the obvious fault in a problematic vacuum system.

So a vacuum gage is a very important indirect component in a successful vacuum system. However, there are many different types of a vacuum “gauge.” The vacuum gauge as shown in Fig. 1 is very familiar, offering a vacuum reading of 0-30”Hg. These gauges are available in various diameters, the most common being from 1” (25mm) through to 4” (100mm). The port connection is either in the center of the rear face or stem-mounted, depending on user preference and positioning in the vacuum line. Glycerin-filled gaug-es are popular particularly on processes with a high cycle frequently as the glycerin liquid dampens the mechanical movement of the bourdon tube assembly. Dry fill gauges will oscillate very quickly in a high-

cycle application, making it difficult to read by the user.

Digital gauges as shown in Fig. 2 are not common-ly used as they are more expensive than the mass-pro-duced bourdon tube type. They are able to withstand more abuse as they have a solid state “mechanism” that is able to withstand considerable oscillating pres-sure spikes and over-pressure scenarios. They are also easier to read for an exact value, but an analogue gauge (dial gauge) offers a quicker reference reading for the user. Digital gauges are often found within a vacuum switch as shown in Fig. 3. These types of switches are available from numerous manufacturers and offer a vacuum reading, but more importantly a vacuum level indication to the PLC of the machinery. This is often used in material handling to ensure a safe vacuum level has been achieved prior to starting the lift cycle.

These switches are available in a variety of guises but fundamentally offer the same features as each other, with or without a readout, preset or adjustable, PNP or NPN operation (PNP sensors are used for the sourcing output, and an NPN sensor is used for the sinking input) and so on. Some models also offer an analogue output to offer the user an actual reading of the vacuum level (a 4-20mA signal converts to a vacu-um level, for example), again which is read by a PLC.

Switches of this type can be used for normally open (NO) or normally closed (NC) operation. Normally closed means that the switch contacts are closed until the vacuum level rises and the contacts open to break the circuit break. “Normally open” means that the switch contacts are broken until the vacuum level ris-es and the contacts come together, creating a circuit. The choice of NC or NO depends on user preference and their intended use.

The switch/sensor shown in Fig. 4 is a typical trans-ducer type model that offers this analogue reading of vacuum level in a system. These types of sensors do not offer a readout, so a dial gauge is often used as

Vacuum Measurement

By Dan Pascoe

Fig. 1

Fig. 2

34 may/june 2011 www.fluidpowerjournal.com | www.ifps.org

at the venturi could be significantly different from the reading taken at the vacuum cups due to length of tubing or poor choice of fittings and other pipe connecting components. Quite often in a centralized system, the venturi is far from the working end of a vacuum-handling system, the vacuum cups. Having a gauge or indeed multiple gauges in the vacuum circuit offers considerable insight into what’s happening in the system and also significant problem-finding advantages.

One of the questions often asked is, “Why won’t my cups pick up the product?” I always ask what vacuum level they are achieving and 90% of the time the answer is, “I don’t know.” Without that information, a prognosis is cer-tainly a more complicated exercise, particularly if this is a “phone-in” question. Vacuum level can quickly determine the obvious fault in a problematic vacuum system.

So a vacuum gage is a very important indirect component in a successful vacuum system. However, there are many different types of a vacuum “gauge.” The vacuum gauge as shown in Fig. 1 is very familiar, offering a vacuum reading of 0-30"Hg. These gauges are available in various diameters, the most

common being from 1" (25mm) through to 4" (100mm). The port connection is either in the center of the rear face or stem-mounted, depending on user preference and positioning in the vacuum line. Glycerin-filled gauges are popular particularly on processes with a high cycle frequently as the glycerin liquid dampens the mechanical movement of the bourdon tube assembly. Dry fill gauges will oscillate very quickly in a high-cycle application, making it difficult to read by the user. Digital gauges as shown in Fig. 2 are not commonly used as they are more expensive than the mass-produced bourdon tube type. They are able to withstand more abuse as they have a solid state “mechanism” that is able to withstand considerable oscillating pressure spikes and over-pressure scenarios. They are also easier to read for an exact value, but an analogue gauge (dial gauge) offers a quicker reference reading for the user. Digital gauges are often found within a vacuum switch as shown in Fig. 3. These types of switches are available from numerous manufacturers and offer a vacuum reading, but more importantly a vacuum level indication to the PLC of the machinery. This is often used in material handling to ensure a safe vacuum level has been achieved prior to starting the lift cycle.

These switches are available in a variety of guises but fundamentally offer the same features as each other, with or without a readout, preset or adjustable, PNP or NPN operation (PNP sensors are used for the sourcing output, and an NPN sensor is used for the sinking input) and so on. Some models also offer an analogue output to offer the user an actual reading of the vacuum level (a 4-20mA signal converts to a vacuum level, for example), again which is read by a PLC.

Often missing from a vacuum

system is something to mea-

sure the vacuum level such

as a vacuum switch or a vac-

uum gauge such as shown in

Fig. 1. A vacuum gauge is a low-cost option for

troubleshooting and production efficiency.

Vacuum pumps are selected based on two

fundamental criteria: flow rate and vacuum level.

The flow rate is determined by understanding

the vacuum level required. Any vacuum pump

or generator (air-powered venturi) will give you

the performance shown in the manufacturer’s

catalogue. However, the flow rate of a pump

or generator will determine if the system can

maintain the desired vacuum level in the actual

application. For example, if there is a leak in the vacuum

circuit, the vacuum level might fall because the

pump or generator cannot “keep up” with the

loss. Imagine sucking through a straw with your

hand on the other end. You can create a vacuum

easily, crushing the plastic straw with ease. If

the straw splits, maintaining a vacuum is a lot

harder. That’s why a larger vacuum flow will be

required to offer extra capacity to compensate

for this leak. If a process requires a certain vacuum level

to fulfill its production design needs, a vacuum

gauge should be installed to offer the user a

visual indication of system performance.

Obvious enough. However, in vacuum lifting

with vacuum cups, more often than not a vacu-

um gauge is not installed. There is rarely a good

reason for this. The standard vacuum gauge as

shown in Fig. 1 is a very low-cost solution to

vacuum measurement. A simple bourdon tube

design, normally manufactured in brass for all

wetted parts (wetted parts refer to the parts of

the gauge in contact with the media—oil, air,

etc) with center back or stem-mounted threads

that connect directly or indirectly to the vacu-

um line. These gauges offer an analogue reading

of the vacuum level at that point in the system.

Placement of these gauge(s) in a vacuum

system is important. Many vacuum generators

offer a vacuum gauge integral to or connected

to the main vacuum port. In centralized

vacuum systems where one venturi is being

used for several vacuum cups, the gauge reading

Fig. 3

Fig. 4

www.ifps.org | www.fluidpowerjournal.com may/june 2011 35

Fig. 7

Fig. 5

www.ifps.org | www.fluidpowerjournal.com may/june 2011 35

Switches of this type can be used for normally

open (NO) or normally closed (NC) operation.

Normally closed means that the switch contacts

are closed until the vacuum level rises and the

contacts open to break the circuit break. “Nor-

mally open” means that the switch contacts

are broken until the vacuum level rises and the

contacts come together, creating a circuit. The

choice of NC or NO depends on user prefer-

ence and their intended use.The switch/sensor shown in Fig. 4 is a typical

transducer type model that offers this analogue

reading of vacuum level in a system. These types

of sensors do not offer a readout, so a dial gauge

is often used as well.Other less expensive switches are available

such as those shown in Fig. 5. This switch

is a mechanical diaphragm type. As the

vacuum level rises, a rubber diaphragm is

drawn back, breaking or making the electrical

circuit. These are very common due to the

lower cost compared to the digital switches

described earlier.  The switching range is

limited compared to digital models, offering

a typical adjustment range between 10"Hg

and 24"Hg. This is ideal for part presence

indication in vacuum cup handling systems.

The hysteresis (see below explanation) is fixed

at about 3"Hg (10% vacuum).

Other large-volume OEM-type switches

such as the model shown in Fig. 6 are low cost

offering a preset switching point and are found

in mass-produced consumer products including

automotive and domestic appliance products.

One of the challenges often associated with

vacuum switches is the hysteresis value that is

more often than not a fixed value on lower-

priced models. Fig. 7 shows a switch that is

purely mechanical but has an adjustable hys-

teresis feature included. Arrow “A” shows the

vacuum range adjustment screw and arrow “B”

shows the hysteresis adjustment screw. This type

of switch is often used on a centralized vacuum

pump system where the pump needs to turn on

and off at specific vacuum levels.

Hysteresis or switching differential is the

difference between the switch position change

value. A normally closed switch could be adjusted

to open at 24"Hg (-80kPa). When the vacuum

level falls (decreases) to 21"Hg, the switch

changes back to its original state (contacts close).

Therefore, this switch has a 3"Hg differential or

hysteresis. This value, as with the switch in Fig. 7,

is sometimes adjustable. 

A vacuum system should contain all the correct components that are required to offer safe operation, long life of components, and reliability for the user. The system should also include, as explained in this overview article, instruments such as gauges and switches that offer the user known values for efficient production throughput and the fault finding tools that enable a quick prognosis for the maintenance engineer.

This article is intended as a general guide and as with any industrial application involv-ing machinery choice, independent professional advice should be sought to ensure correct selection and installation.

Daniel Pascoe is General Manager of Vacuforce Inc., manufacturer and distributor of vacuum components and systems for industry in North America. Daniel can be reached via the Vacuforce Web site at www.vacuforce.com or directly at dpascoe@vacuforce.com. You can also find Vacuforce on Facebook.

Fig. 6